Notch filters, also commonly known as band-stop or band-rejection filters, reject a particular band of frequencies. Notch filters are also known as band elimination filters since they eliminate frequencies. The characteristics of a notch filter are essentially the inverse of the characteristics of a band pass filter. A notch filter has two cut-off frequencies (i.e. lower and upper cut-off frequencies) unlike high pass and low pass filters. The notch filter has two pass bands and one stop band. The notch filter passes signals above and below a determined range of frequencies (stop-band) and attenuates frequencies in between the cut-off frequencies.
Signal impurities naturally occur in radio frequency transmission technologies. These signal impurities, also known as spurious emissions, spurious harmonics, spurious signals, parasitic emissions, etc. are attenuated to reduce the effect on the transmission of corresponding data. The more spurious harmonics that are present in a frequency band, the fewer frequencies are available for use, e.g., for data transmission, cellular applications, radio transmission application, etc.
One technique to remove or attenuate spurious harmonics is to design wide band antennas to have narrow rejection bands. Alternatively, band-pass filters (BPFs) can be designed with single or multi narrow rejection bands. In general, this can be achieved by adding transmission line elements, such as conventional open-circuited stubs, whose electrical length is a quarter wavelength at the desired center frequency of the notched band. The characteristic impedance of the open-circuited stub is determined by the width of the structure.
The bandwidth of the notched band is directly proportional to the width of the open circuited stubs. Therefore, the physical width of the open circuited stub W becomes very small and difficult to fabricate, using conventional low cost printed circuit-board (PCB) technology, when narrow bandwidth is required. In addition, this technique increases the overall size of the design circuit board. To overcome these problems and to achieve a narrow notched band with realizable physical dimensions and small circuit size, spur lines and embedded open-circuited stubs can be implemented instead of conventional open-circuited stubs. The even and odd modes characteristic impedances of the spur line and embedded open-circuited stub are determined by the width and the gap which can be used to control the bandwidth of the notch.
Since spur lines and embedded open-circuited stubs are embedded into other components such as input and output feed lines, a notch can be generated without increasing the size of the circuit board. On the other hand, embedded open-circuited stub makes it possible to realize very high impedance. Hence, a very narrow rejection band can be achieved. However, the conventional open-circuited stub, spur line, and embedded open-circuited stub, whose electrical length is about a quarter wavelength long at the desired center frequency, have their spurious second harmonic at three times the center frequency of the notched band due to their distributed behavior. Since ultra-wide band (UWB) radio signals can cover a very wide band of frequency, i.e., from 3.1 gigahertz (GHz) to 10.6 GHz, the second harmonic might appear within the UWB allocated spectrum. For example, for WiMAX applications operating at the 3.5 GHz, the second harmonic when using conventional distributed components can appear at or below 10.5 GHz.